Katzung 12th ed - Chapter 25 - General Anaesthetics - Inhaled Anaesthetics (1)

Question 1

Name the five primary effects of general anaesthetics that are useful.

Answer 1

• Unconsciousness
• Amnesia
• Analgesia
• Inhibition of autonomic reflexes
• Muscle relaxation

Question 2

Name the two broad groups within the inhaled anaesthetics.

Answer 2

Volatile and Gaseous.

Question 3

What is meant by MAC when discussing inhaled anaesthetic agents?

Answer 3

MAC is used to describe the potency of an anaesthetic agent. An agent’s MAC 1.0 is the Minimal Alveolar Concentration (partial pressure of the drug in the alveoli) at which 50% of the population would remain immobile at the time of first skin incision in surgery (without giving them any other relaxants / sedatives).

Question 4

Name the four stages of general anaesthesia (Guedel’s)

Answer 4

Stage 1 - analgesia
Stage 2 - excitement
Stage 3 - surgical anaesthesia
Stage 4 - medullary depression

Question 5

How does a volatile agent’s solubility affect its distribution to the brain?

Answer 5

The more soluble an agent, the longer it takes for its partial pressure in blood to rise, and thus the slower the onset of anaesthesia.

Question 6

What is MAC 1.0 of nitrous oxide?

Answer 6

> 100% (this means that even if the partial pressure of NO in the alveoli is 100%, surgical anaesthesia will still not be achieved)

Question 7

Describe the basic pharmacodynamics of NO, halothane, methoxyfluorane, and isofluorane.

Answer 7

These cause decreased neuronal activity. The mechanism involves hyperpolarization by opening potassium channels.

Question 8

What is meant by the blood:gas partition coefficient?

Answer 8

This is the solubility of an inhaled anaesthetic agent. It refers to the ratio of [concentration in blood] to [concentration in gas in alveolus].

Question 9

Name one inhaled anaesthetic agent with a low blood:gas partition coefficient.

Answer 9

NO with 0.47 (this means the solubility is low)

Question 10

Name one or two inhaled anaesthetic agents with a high blood:gas partition coefficient.

Answer 10

Halothane with 2.3, or methoxyfluorane with 12 (this means the solubility is high)

Question 11

Write these inhaled anaesthetic agents in order from highest MAC 1.0 to lowest MAC 1.0:

• Methoxyfluorane
• Isofluorane
• Nitrous oxide
• Halothane

Answer 11

• Nitrous oxide: >100%
• Isofluorane: 1.4%
• Halothane: 0.75%
• Methoxyfluorane: 0.16%

Question 12

Which organ contributes the most to the metabolism of inhaled anaesthetic agents?

Answer 12

Lungs.

Question 13

Describe two rare but important adverse effects of inhaled anaesthetic agents.

Answer 13

Hepatic necrosis (1 in 35,000 having halothane)
Malignant hyperthermia (multiple inhaled agents): tachycardia, hypertension, hyperthermia, acidosis, muscle rigidity.

Question 14

What is meant by the brain:blood partition coefficient?

Answer 14

This refers to the degree to which the drug is taken up by the brain from the blood. It is the ratio of [concentration in brain] to [concentration in blood]. It is >1 for all inhaled anaesthetic agents.

Question 15

Write these inhaled anaesthetic agents in order from least metabolized to most metabolized:

• Halothane
• Isofluorane
• Nitrous oxide
• Methoxyfluorane
• Desfluorane
• Sevofluorane

Answer 15

• Nitrous oxide (zero metabolized)
• Desfluorane
• Isofluorane
• Sevofluorane (2-5%)
• Halothane (>40%)
• Methoxyfluorane